Leveillula Taurica on Potatoes

First Report of Powdery Mildew on Potato Caused by Leveillula taurica in North America

Dean A. Glawe, Plant Pathologist, Puyallup Research and Extension Center, Washington State University, 7612 Pioneer Way East, Puyallup 98371-4998; Lindsey J. du Toit, Assistant/Extension Plant Pathologist, Northwest Washington Research and Extension Center, Washington State University, 16650 State Route 536, Mount Vernon 98273-4768; and Gary Q. Pelter, Washington State University Extension, County Courthouse, P.O. Box 37, Ephrata 98823

Corresponding author: Dean A. Glawe. [email protected]

Glawe, D. A., du Toit, L. J., and Pelter, G. Q. 2004. First report of powdery mildew on potato caused by Leveillula taurica in North America. Online. Plant Health Progress doi:10.1094/PHP-2004-1214-01-HN.

Powdery mildew causes losses in potato (Solanum tuberosum L.) production throughout much of the world (3,4). In the Columbia Basin of Washington, this disease appears most damaging in potatoes grown under furrow irrigation (3). The causal agent usually has been reported to be Erysiphe cichoracearum DC. (3), sometimes referred to as Golovinomyces cichoracearum (DC.) V.P. Gelyuta (4). In his world monograph, Braun (1) subdivided E. cichoracearum into several species, restricted the use of E. cichoracearum to species on Asteraceae, and used Erysiphe orontii Castagne for the segregate on Solanaceae (1). In the Middle East a different pathogen, designated as Oidiopsis (anamorphic state of a Leveillula species), also was reported on potato (2). Chorin (2) found that the two powdery mildew species occurred independently or on the same plant. During August 2004, examination of powdery mildew-infected ‘Russet Burbank’ potato leaves from a furrow-irrigated field in Grant County, WA, revealed two powdery mildew fungi, one referable to E. orontii and the other to Leveillula taurica (Lév.) G. Arnaud (1). This report documents, for the first time, L. taurica on potato in North America and provides information on distinguishing it from E. orontii. Diseased leaves displayed localized, chlorotic to necrotic regions with typical powdery mildew signs consisting of whitish masses of conidia and conidiophores. Examination of periclinal, free-hand sections of leaf tissue revealed that conidiophores of the two fungi were intermixed. Erysiphe orontii formed superficial hyphae with nipple-shaped appressoria (Fig. 1), conidiophores with conidial chains (Fig. 2), and ovoid to short-cylindrical conidia lacking fibrosin bodies and measuring (24.0-) 27.0-33.0 (-35.5) × (13.0-) 14.0-18.0 (-19.0) µm. Leveillula taurica formed conidiophores (Fig. 3) emerging through stomata from internal mycelium, with single conidia that were lanceolate or cylindrical and measured (37.0-) 43.5-58.0 (-58.5) × 11.0-17.0 (- 18.5) µm. Conidia and conidiophores of both fungi were observed together in microscopic slide mounts in which the conidia were distinguishable on the basis of size and shape (Fig. 4). The majority (> 90%) of conidia and conidiophores observed were produced by E. orontii.

Plant Health Progress 14 December 2004

Fig. 1. Erysiphe orontii appressoria viewed Fig. 2. Erysiphe orontii with brightfield microscopy. conidiophore with four conidia, adjacent to plant trichome, viewed with stereomicroscopy.

Fig. 3. Leveillula taurica Fig. 4. Short-cylindrical to conidiophore with single, ovoid conidia of Erysiphe cylindrical conidium viewed orontii (E), and cylindrical with stereomicroscopy. and lanceolate conidia of Leveillula taurica (L) viewed with brightfield microscopy.

Discovery of the two species sporulating together on diseased leaves is consistent with the similar observation made in the Middle East (2). The preponderance of E. orontii conidiophores and conidia tended to obscure the presence of L. taurica. It is possible that L. taurica has been overlooked in the past because its presence was masked by profuse sporulation of E. orontii. Easton and Nagle (3) noted that although application of sulfur dust or liquid for control of potato powdery mildew generally was successful, the applications sometimes failed to prevent development of severe symptoms. One possible explanation for variable effectiveness of fungicide treatments is that L. taurica and E. orontii might differ in their sensitivity to fungicides. Further research is warranted to assess the co-occurrence of L. taurica and E. orontii on potato crops in North America, the factors governing co-infection, and whether responses to foliar fungicide applications by the two pathogens are similar.

Plant Health Progress 14 December 2004

Literature Cited 1. Braun, U. 1987. A monograph of the Erysiphales (powdery mildews). Beih. Nova Hedwigia 89:1-700. 2. Chorin, M. 1946. The powdery mildews of potatoes in Palestine. Palestine J. Bot. Hort. Sci. Ser. R. 5:259-261. 3. Easton, G. D., and Nagle, M. E. 1990. Aircraft-applied sulfur for control of powdery mildew (Erysiphe cichoracearum) on potato in Washington State. Amer. Potato J. 67:385-392. 4. Johnson, D. A., and Rowe, R. C. 2001. Powdery mildew. Pages 34-35 in: Compendium of Potato Diseases, 2nd ed. W. R. Stevenson, R. Loria, G. D. Franc, and D. P. Weingarter, eds. American Phytopathological Society, St. Paul, MN. 5. Romberg, M. K., Nuñez, J. J., and Farrar, J. J. 2004. First report of powdery mildew on potato caused by Golovinomyces cichoracearum in California. Plant Dis. 88:309.

Plant Health Progress 14 December 2004